As semiconductor geometries continue to shrink, manufacturers have increasingly turned to optical techniques to perform non-destructive inspection and analysis of semiconductor wafers. Techniques of this type, known generally as optical metrology, operate by illuminating a sample with an incident field (typically referred to as a probe beam) and then detecting and analyzing the reflected energy. Ellipsometry and reflectometry are two examples of commonly used optical techniques. For the specific case of ellipsometry, changes in the polarization state of the probe beam are analyzed. Reflectometry is similar, except that changes in intensity are analyzed. Scatterometry is a specific type of optical metrology that is used when the structural geometry of a sample creates diffraction (optical scattering) of the probe beam. Scatterometry systems analyze diffraction to deduce details of the structures that cause the diffraction to occur.
Various optical techniques have been used to perform optical scatterometry. These include broadband spectroscopy (U.S. Pat. Nos. 5,607,800; 5,867,276 and 5,963,329), spectral ellipsometry (U.S. Pat. No. 5,739,909) single-wavelength optical scattering (U.S. Pat. No. 5,889,593), and spectral and single-wavelength beam profile reflectance and beam profile ellipsometry (U.S. Pat. No. 6,429,943). Scatterometry, in these cases generally refers to optical response information in the form of diffraction orders produced by periodic structures (e.g., gratings on a wafer). In addition it may be possible to employ any of these measurement technologies, e.g., single-wavelength laser BPR or BPE, to obtain critical dimension (CD) measurements on non periodic structures, such as isolated lines or isolated vias and mesas. The above cited patents and patent applications, along with PCT Application WO03/009063, US Application 2002/0158193, US Application 2003/0147086, US Application 2001/0051856 A1, PCT Application WO 01/55669 and PCT Application WO 01/97280 are all incorporated herein by reference.
As shown in FIG. 1, a typical optical metrology system includes an illumination source that creates a monochromatic or polychromatic probe beam. The probe beam is projected by one or more lenses onto the surface of a sample. The portion of the sample surface that is illuminated by the probe beam is referred to as the illumination spot. The sample reflects the probe beam and the reflected probe beam (or a portion of the reflected probe beam) is transported to a detector. For most systems, the detector gathers energy from a small area within the sample surface known as the measurement spot. The measurement spot may be smaller or larger than the illumination spot. The detector transforms the energy it receives into corresponding output signals. A processor analyzes the signals to measure the structure or composition of the sample. For cases where the probe beam is polychromatic, the detector is often preceded by an optical element that disperses the incoming probe beam into different wavelengths. A prism or grating may be used for this purpose. This allows different portions of the detector to measure different spectral components of the reflected probe beam off the sample. In these cases, the combination of the spreading optical component and the detector is generally referred to as a spectrometer.
The decreasing size of semiconductor geometries forces metrology to analyze increasingly small areas. In practice, the area being measured is typically a test feature that is often less than 50 μm wide and is often surrounded by a completely different material or film stack. Accurately analyzing small areas requires that the illumination spot and the measurement spot be relatively small. This, in turn requires that the incoming and reflected probe beams be tightly focused to support small spot sizes.
One way of meeting these requirements (at least partially) is by using small angles of incidence for the incoming and reflected probe beams. Small angle of incidence systems reduce the amount by which the sample projects the illumination and measurement spots. This is one of the main motivations behind metrology systems of the type shown in FIG. 2 in which the incoming and reflected probe beams are directed normally to the sample surface. As may be appreciated, the use of normal incidences means that the illumination and measurement spots have a circular cross-section. The circular cross-section is effectively smaller than the elliptical cross-section associated with non-normally directed beams.
In systems where normal incidence is used (e.g., the system of FIG. 2) the focusing assembly transports both the incident and the reflected probe beam. This differs from non-normal systems where distinct optical components are used for incident and reflected probe beams. Several focusing assemblies have been developed for this purpose. These focusing assemblies can be formed from refractive or reflective elements or a combination of refractive and reflective elements. Of the different designs, refractive designs are attractive because they have no central and peripheral [e.g., spider arms] obscurations that may cause scattering within the metrology tool. Unfortunately, chromatic aberrations are inherent in refractive optics. Minimizing such aberration over the wavelength range of DUV to IR is difficult while maintaining small spot sizes.
It is another common practice to use rotationally symmetric reflective designs such as Schwarzschild microscope objectives. As shown in FIG. 3, a classical Schwarzschild objective consists of a large primary concave and a small secondary convex mirror system in which the two mirrors are or nearly monocentric. The two mirrors form a path between an illumination source or a detector (or both) and the surface of a sample. Unfortunately, the secondary mirror forms a central obscuration and secondary mirror hardware support structures form spider arm obscurations. The central obscuration blocks useful probe beam at and near zero numerical aperture and increases diffraction effects. Further, the spider arm obscurations introduce scatters as a function of spider arms geometry which is undesirable when measuring patterned structures.